The objective of this research is to develop the apparatus and technique by which microwave thermal energy applied externally over the precordium may be transmitted through the chest wall to warm atrial or ventricular pacemaker cells in selected areas of the heart so as to arouse, accelerate, and maintain intrinsic rhythms at clinically useful rates and time intervals. A microwave generator and matched applicator will be developed with use of solid-state microwave technique, computer simulations, optimization procedures, and modern fabrication technologies so that the depth of penetration, volume of tissue irradiated, and temperature change are well controlled. The distribution, degree, and temporal course of heating of the target areas of the heart and of intervening tissues will be determined in normal anesthetized dogs by appropriate placement of thermistor probes. In dogs with surgically induced AV block the changes in atrial and ventricular rhythmicity will then be studied. In particular, acceleration of the normal sino-atrial node and of the dominant idioventricular pacemaker and the arousal, acceleration, and maintainance of ectopic atrial and ventricular pacemaker will be examined. Thermal effects on cardiac metabolism and hemodynamics will also be observed. Such as technique of temporary control of cardiac rhythm that is quick, effective, entirely safe and painless, and completely noninvasive may supplement and at times even supplant present-day methods involving external cardiac pacemakers and intravenous drugs.